Re: [Xen-devel] QEMU bumping memory bug analysis

[George Dunlap <george.dunlap@xxxxxxxxxxxxx>]

On 06/09/2015 11:14 AM, Stefano Stabellini wrote:
> On Mon, 8 Jun 2015, George Dunlap wrote:
>> I think that qemu needs to tell libxl how much memory it is using for
>> all of its needs -- including option ROMs.  (See my example below.)  For
>> older qemus we can just make some assumptions like we always have.
> 
> I'll just note that I have still not seen any evidence that telling
> libxl would improve things. It seems to me that we are just adding one
> more layer of indirection just to solve the migration issue elsewhere.
> I haven't seen convincing examples that things would be better by telling
> libxl yet, except that we would be able to fix the migration problem.

So from a migration perspective, I agree there's not much difference
between "libxl reads max_pages from Xen, inserts it into the migration
stream, and sets it again on the other side" and "libxl has global
knowledge of what max_pages should be, inserts it into the migration
stream, and sets it again on the other side".

Improvements of having it in libxl:

1. libxl is not an external interface; we can change and improve things
as we go along.  Whatever we put in qemu we have to support indefinitely.

2. We can begin to solve the "where is the memory" mess that is the
current state of things.

3. In particular, we could conceivably actually change the interface to
be consistent, so that "memory" means "the maximum amount of memory used
by the guest including all overhead", rather than the random who knows
what we have now.  This will be impossible if we do it in qemu.

Having more than one place change max_pages would be poor design even if
we were still packaging everything together in the same release.  Having
an external entity with which we must be backwards-compatible change
max_pages it is just a bad idea and always was.

> When I committed it, I didn't
> do it without thinking. I don't think that libxl should be in the middle
> of this, because I don't think it has anything to add.

Just to be clear, nobody thinks you did (at least, as far as I know).
The problem is that all of us are so specialized: some people work
almost entirely within qemu, others in the kernel, others in the
toolstack, others in Xen.  It's just natural for people who work mainly
within one component to look at the problem from a particular
perspective.  But it causes exactly this sort of problem, where the left
hand doesn't know what the right hand is doing, and the overall system
is really uncoordinated.

 -George

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